This invention relates to the field of synthesis of aminophenols and more particularly to an improved process for the preparation of substituted or unsubstituted p-aminophenol through the rearrangement of a hydroxylamine produced by partial hydrogenation of nitrobenzene.
In the conventional commercial process for manufacture of p-aminophenol, as described in Benner U.S. Pat. No. 3,383,416, nitrobenzene is catalytically hydrogenated in the presence of a dilute sulfuric acid solution. Under the conditions utilized in the Benner patent, the nitrobenzene is subject to complete reduction to aniline by reaction with three moles of hydrogen: ##STR1## However, the intermediate phenylhydroxylamine, produced by reaction of nitrobenzene with two moles of hydrogen, is rapidly removed from the reaction zone by absorption into the sulfuric acid phase where it rearranges to p-aminophenol: ##STR2## Fortuitously, the noble metal catalyst congregates in the nitrobenzene phase so that phenylhydroxylamine absorbed into the acid phase is essentially removed from the reaction zone, and conversion of phenylhydroxylamine to aniline is thereby inhibited. This feature also permits recovery of the catalyst in the nitrobenzene phase when the phases are permitted to separate so that, by terminating the reaction prior to the conversion of all of the nitrobenzene, the catalyst is easily recovered by decantation and recycled in its nitrobenzene vehicle for use in another batch or in an earlier stage of a cascade continuous hydrogenation system.
Despite its numerous salutary features, the Benner process does not entirely avoid conversion of phenylhydroxylamine to aniline. Thus, typically the yield of p-aminophenol is about 75% while 15% of the nitrobenzene is converted to aniline. The amount of byproduct aniline obtained can be minimized by lowering the reaction rate but this, of course, involves a sacrifice in productivity.
Rylander et al., U.S. Pat. No. 3,694,509 describes a method for producing arylhydroxylamines while minimizing the fraction of arylamines that are produced prior to the uptake of two moles of hydrogen per mole of substrate. Rylander et al achieve this improvement by incorporating from 0.1 to 100 mols dimethyl sulfoxide per mole of catalyst metal in the reactor charge mixture. However, Rylander et al.'s disclosure is concerned only with the preparation of arylhydroxylamines and contains no teaching relevant to the production of p-aminophenol by catalytic hydrogenation or otherwise.
Rylander et al. U.S. Pat. No. 3,715,397 discloses a process for preparation of p-aminophenol by catalytic hydrogenation of nitrobenzene in a sulfuric acid system in the presence of DMSO, using a platinum oxide catalyst. Phenylhydroxylamine produced in the hydrogenation is converted in situ to p-aminophenol. The acid strength is that obtained by mixing 25 ml concentrated sulfuric acid with 75 ml water. Reaction is carried out in a batch system until absorption of 2 moles of H.sub.2 per mole of nitrobenzene.
Japanese Pat. No. 54[1979]-24837 discloses a process for producing arylhydroxylamines by catalytic hydrogenation of substituted or unsubstituted nitrobenzene in a solvent containing phosphoric acid, its alkali metal salts, an ester phosphite, an ester thiophosphite, an alkyl or aryl phosphine, an alkyl or aryl aminophosphine, an alkyl or aryl sulfide, a carboalkoxy alkyl sulfide, an alkyl or aryl mercaptan or thiophene.